As a method for manufacturing an SOI wafer, especially for manufacturing a thin-film SOI wafer that can improve the performance of an advanced integrated circuit, a method for delaminating an ion-implanted wafer after bonding to manufacture an SOI wafer (an ion-implantation delamination method: technology that is also called the Smart Cut method (a registered trademark)) has been received attention.
This ion-implantation delamination method is technology that forms an oxide film on at least one of two wafers, implants gas ions such as hydrogen ions or rare gas ions from an upper surface of one silicon wafer (a bond wafer), forms a micro bubble layer (a sealed layer) in the interior of the bond wafer, then adhering the surface from which the ions are implanted to the other silicon wafer (a base wafer) through the oxide film, subsequently performs a heat treatment (a delamination heat treatment) to delaminate the bond wafer into a thin film-form by making the micro bubble layer be a cleaved surface (a delaminated surface), and further performs a heat treatment (a bonding heat treatment) to strengthen the bonding, thereby manufacturing an SOI wafer (see Patent Document 1). In this stage, the cleaved surface becomes a surface of a SOI layer, and an SOI wafer having a thin SOI film with high thickness uniformity can be obtained relatively easily. When obtaining a bonded wafer in which wafers are directly bonded, instead of an SOI wafer, the bond wafer and the base wafer are directly adhered and bonded with each other without forming an oxide film onto either surface thereof.
The surface of the SOI wafer after delamination, however, contains a damaged layer due to ion implantation, and has surface roughness larger than that of a mirror surface of a regular silicon wafer. Accordingly, in the ion-implantation delamination method, such a damage layer or surface roughness have to be removed.
Conventionally, to remove this damage layer or the like, mirror polishing with a very small polishing stock removal (a stock removal: approximately 100 nm), which is called touch polish, is carried out at a final step after the bonding heat treatment. However, when the SOI layer is subjected to polishing including machine work, since the polishing stock removal is not uniform, there arises a problem of deteriorating film thickness uniformity of the SOI layer achieved by implantation of hydrogen ions, etc. and delamination.
As a method for solving such a problem, a high temperature heat treatment are displacing the aforementioned touch polishing to perform a flattening treatment that improves surface roughness.
For example, Patent Document 2 proposes a method of performing a heat treatment (a rapid heating/rapid cooling heat treatment (an RTA treatment)) in a reducing atmosphere containing hydrogen without polishing a surface of an SOI layer after a delamination heat treatment (or a bonding heat treatment). Patent Document 3 proposes a method in which an oxide film is formed onto an SOI layer by a heat treatment in an oxidizing atmosphere after a delamination heat treatment (or a bonding heat treatment), and then the oxide film is removed (a sacrificial oxidation treatment), followed by performing an RTA treatment in a reducing atmosphere. Patent Documents 4 to 6 propose a method of performing a sacrificial oxidation treatment after performing an RTA treatment, and a method in which the plural number of RTA treatments and sacrificial oxidation treatments are performed.
In manufacturing an SOI wafer by an ion-implantation delamination method, it is usual to perform a treatment to adjust the film thickness of the SOI layer to an intended film thickness such as a sacrificial oxidation treatment or polishing after performing a heat treatment to flatten the delaminated surface such as the foregoing RTA treatment and so on (Patent Documents 4 to 6).
Before put into such a treatment to adjust the film thickness, an SOI wafer is usually subjected to cleaning process. In the cleaning process, it is usual to perform cleaning in which an SOI wafer is dipped into cleaning liquid which functions to slightly etch the surface of an SOI layer such as SC1 (mixed solution of NH4OH and H2O2).